Research utility of noninvasive methods for measurement of cardiac output

Two noninvasive methods of cardiac index (Q) determination, pulsed Doppler echocardiography (QDop), and CO2 rebreathing (QCO2) were compared to dye dilution in eight normal volunteers. Measurements of Q were made by dye dilution (QDD) and the two noninvasive techniques under the following conditions: supine rest, after inflation of cuffs around the thighs, 35-degree head-up tilt, supine rest repeated, during constant isoproterenol infusion, and after an intravenous bolus of propranolol. When mean Q values of the group for each intervention were compared, close agreement was observed between dye dilution and each noninvasive method (QDOP = 1.08 QDD - 0.07 L/min X m2, r = 0.99, SEE = 0.02; QCO2 = 0.68 QDD + 0.84 L/min X m2, r = 0.97, SEE = 0.02). Fair correlations were achieved when all 48 paired observations were analyzed (QDop = 1.00 QDD + 0.17 L/min X m2, r = 0.89, SEE = 0.17; QCO2 = 0.71 QDD + 0.77 L/min X m2, r = 0.79, SEE = 0.20). However, the Doppler ultrasound technique was significantly more precise in quantitating changes in Q in individual patients. These results demonstrate that estimates of Q can be made with reasonable accuracy by either CO2 rebreathing or Doppler echocardiography. However, the Doppler technique is a much more accurate means of quantitating acute changes in Q.